The stability and plasticity of synapses are both important for cognitive functions. Plasticity is necessary for learning, while stably formed synapses are believed to encode for long term memory. Yet it is unclear how neurons can maintain this delicate balance and achieve both stability and plasticity in synapses just a few microns apart. Clearly, a high level of compartmentalization is required, which makes polarity proteins ideal candidates to function in this capacity, as they serve to separate and maintain distinct membrane domains. Indeed, we previously showed that a class of polarity proteins called Par (partitioning defective) proteins plays a key role in dendritic spine morphogenesis, with Par1, Par3 and Par6 important for this process. Interestingly, our preliminary studies show that most of the spines in mature neurons are dominated by Par3 and Par6, whereas a smaller fraction of spines are dominated by Par1. This distribution pattern resembles that of Par proteins in developing zygotes and epithelial cells, where Par1 and the Par3/6 complex show opposite localization by mutually excluding each other from their respective domains. Furthermore, our previous studies suggest that Par3 and Par6, which form a complex with atypical PKC (aPKC), promote spine stability and maturation, whereas our preliminary results suggest that Par1 promotes spine plasticity. These data raise the exciting possibility that the Par3/6 complex and Par1 regulate the balance between stability and plasticity of spines, with Par3/6-dominant spines being more stable and Par1-dominant spines being more plastic. We plan to test this overarching hypothesis through two aims. In aim1, we will test the hypothesis that Par1 promotes synaptic plasticity and that plasticity is maintained by increasing the dynamics of the PSD scaffold and by excluding the Par3/6 complex. In aim 2, we will test the hypothesis that the Par3/Par6/aPKC complex promotes synaptic stability and that stability is maintained in part by excluding Par1 from the spines. We will utilize advanced molecular imaging techniques, including FRET, FRAP, fluorescent light-inducible proteins (FLIPs) and 2-photon glutamate uncaging. In addition, we will use 2-photon imaging in live mice to directly test our hypothesis in behaving animals. We will combine these imaging approaches with biochemical analysis as well as electrophysiology. This interdisciplinary approach will allow us to gain fundamental insight into the cellular and molecular mechanisms of synaptic stability and plasticity. Moreover, multiple lines of evidence are pointing to an important role for Par polarity proteins in neuropsychiatric disorders, including schizophrenia, autism, major depressive disorder and bipolar disorder. Thus, our studies will provide mechanistic insight into synaptic plasticity and stability and may shed light on several devastating abnormalities that affect the human brain.